In recent years, as a technique other than water-based or solvent-based inkjet print, inkjet print using an ultraviolet-curable resin has been studied. Since this ultraviolet-curable resin has a feature that the resin is cured instantly by ultraviolet ray irradiation, it has a merit that no ink absorbing layer is required for a recording base material. Because of this merit, usage of the above-described ultraviolet-curable resin has been studied as a coloring material for applications not only coloring on paper but also on various raw materials such as film, plastic, metal and glass.
Since the ultraviolet-curable resin becomes a hardened film excellent in scratching hardness and adhesion to a base material, the recorded matter can be used both indoors and outdoors. However, being compared with indoor use, in the case of outdoor use such as building materials and advertising displays for example, excellent weather resistance being able to respond to all natural conditions is necessary. Weather resistance is resistance specifically when exposed to heat, light, water and so forth. The recorded matter must be kept for a certain period of time even under such environment without changing color of printed image and degrading the printed image, and an ink layer must not crack or exfoliate from a base material. In general, ultraviolet-curable resins are superior to other resins in weather resistance, but not sufficient yet.
Therefore, inks for inkjet in outdoor use have been the subject of daily study, and as a coloring material, use of organic pigments superior to dyes in light resistance has just started. However, in an outdoor application, even in the case that organic pigments are used, color degradation cannot be avoided because of exposure to sunlight for a long period of time. In an outdoor application, it is necessary to have light resistance such that the coloring suffers almost no color degradation after being exposed for 5 to 10 years outdoors.
Hence, there is a method of using inorganic pigments with more excellent light resistance than organic pigments as a coloring agent. For example, in Japanese Unexamined Patent Publication No. 2001-55530, there is disclosed a color printing method which uses an inorganic composite oxide pigment, and has excellent light resistance and weather resistance, and further has an excellent visibility of hue. However, in this method, although light resistance is improved because of using an inorganic pigment, a printed matter with high-concentration cannot be obtained because an inorganic pigment is inferior to an organic pigment in coloring power.
The inorganic composite oxide pigment used there tends to be pale in color due to possible breakdown of crystalline structure when excessively dispersed. Above all, since a yellow inorganic composite oxide pigment and a blue inorganic composite oxide pigment are poor in coloring power, when color concentration is tried to be increased, a very large amount of pigment is required, from which an inkjet ink cannot be realized. Thus, substantially it becomes difficult to make ink color concentration higher. Since inorganic composite oxide pigments has no brilliant red color, there is a problem that representation of neutral color from yellow to red, orange color for example, or representation of neutral color from red to blue, purple color for example is poor.
Generally for obtaining a high-concentration printed matter, solid content of pigment in an ink may be increased, but the compoundable amount of reactive monomer and/or reactive oligomer decreases, and the ink becomes viscous, making the discharge difficult, thereby clogging of nozzles may occur as well. Therefore, there is a limitation in the increment of the solid content of pigment in an ink for inkjet printing.
In this way, there has not been developed yet an ultraviolet-curable inkjet ink set capable of conducting high-concentration printing and obtaining a brilliant printed matter being excellent in color representation and weather resistance.